how to take an input by user in javascript code example

Example 1: js get input from user

var name = prompt("Please enter your name", "Harry Potter");

Example 2: user input through js

var at = document.getElementById("email").value.indexOf("@");
var age = document.getElementById("age").value;
var fname = document.getElementById("fname").value;
submitOK = "true";

if (fname.length > 10) {
  alert("The name may have no more than 10 characters");
  submitOK = "false";
}

if (isNaN(age) || age < 1 || age > 100) {
  alert("The age must be a number between 1 and 100");
  submitOK = "false";
}

if (at == -1) {
  alert("Not a valid e-mail!");
  submitOK = "false";
}

if (submitOK == "false") {
  return false;
}